This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
A wireless LAN (WLAN) is a flexible data communications system implemented as an alternative or extension to a wired LAN within a building or campus. Using electromagnetic waves, WLANs transmit and receive data over the air, minimizing the need for wired connections. Thus, WLANs combine data connectivity with user mobility, and, through simplified configuration, enable movable LANs. Some industries that have benefited from the productivity gains of using portable terminals (e.g., notebook computers) to transmit and receive real-time information are the digital home networking, health-care, retail, manufacturing, and warehousing industries.
Manufacturers of WLANs have a range of transmission technologies to choose from when designing a WLAN. Some exemplary technologies are multicarrier systems, spread spectrum systems, narrowband systems, and infrared systems. Although each system has its own benefits and detriments, one particular type of multicarrier transmission system, orthogonal frequency division multiplexing (OFDM), has proven to be exceptionally useful for WLAN communications.
OFDM is a robust technique for efficiently transmitting data over a channel. The technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit data. These sub-carriers are arranged for optimal bandwidth efficiency compared to conventional frequency division multiplexing (FDM) which can waste portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier.
The transmission of data through a channel via OFDM signals also provides several other advantages over more conventional transmission techniques. Some of these advantages are a tolerance to multipath delay spread and frequency selective fading, efficient spectrum usage, simplified sub-channel equalization, and good interference properties.
In spite of these advantages, there are some problems with OFDM data transfer in systems that are adapted to process multiple signals from multiple users. One example of such a problem is adjacent channel interference (sometimes referred to as interchannel interference). One of the main reasons for adjacent channel interference is the windowing that occurs as an inherent part of the Fast Fourier Transform (FFT) processing of signals that is typical of all OFDM receivers. In processing signals using FFT algorithms, the tone of each OFDM sub-band may be spread across multiple FFT bins with periodically spaced zero crossings (e.g. a sinc function in the case of a rectangular window) in the frequency domain. Such a frequency domain structure allows adjacent channels to be placed at the multiples of the sampling rate with no adjacent channel interference because of the alignment of the zero crossing. Thus, even though the energy from an adjacent channel in the frequency range for the desired channel is non-zero, there is no adjacent channel interference because of the orthogonal nature of the two signals.
However, when a carrier frequency offset is present, the orthogonality between adjacent channel signals is not maintained and adjacent channel interference occurs. A method of reducing adjacent channel interference in OFDM receivers is desirable.